Unveiling how THF tunes the micro-kinetics of H₂-THF clathrate for enhanced H₂ storage: A molecular dynamics study

Hydrogen (H₂) as a clean energy carrier can be effectively stored in clathrate hydrates with the aid of thermodynamic promoters. Tetrahydrofuran (THF), as the most classical thermodynamic promoter is effective at enhancing H₂ storage capacity when used at sub-stoichiometric concentrations. However, the mechanisms of THF concentration (C_THF) on tuning H₂-THF hydrate formation kinetics and cage occupancy remain less understood. In this study, we employ molecular dynamics simulations to systematically investigate the effects of C_THF (1.0-5.6 mol%) on H₂-THF hydrate growth kinetics and H₂ cage occupancy at 50 MPa and 250 K. Increasing C_THF enhances water molecule ordering reflected by the increase in the F₄ order parameter from 0.31 to 0.65. At optimal 3.5 mol%, THF near the hydrate growing interface promotes water diffusion and facilitates sII hydrate cage growth, leading to initial enhanced H₂-THF hydrate growth kinetics. In contrast, excessive THF accumulation at 5.6 mol% hinders water diffusion and suppresses H₂-THF hydrate cages growth, while insufficient THF at 1.0 mol% restrains the formation of 5¹²6⁴ cages. Cage occupancy analysis reveals that reducing C_THF decreased H₂ occupancy in 5¹² small cages from 78.8 % to 72.2 % but increased that in 5¹²6⁴ large cages reaching a maximum of 43.6 % at 3.5 mol% THF. Moreover, double H₂ occupancy is identified in 5¹²6⁴ cages at all C_THF but only remains stable for ∼100 ns. H₂ storage capacity exhibits a non-monotonic trend, peaking at 0.79 wt% for 3.5 mol% THF. These insights advance understanding of the tuning effects of C_THF on H₂-THF hydrates at the molecular level and guide the use of thermodynamic promoters at optimal concentrations for hydrate-based H₂ storage.